There is no option to un-filter. Since it's not clear from your question, whether you are asking from the perspective of a business-owner, review-writer or review-reader, I will cover all three:
However, as a review-writer whose review for a particular business has been filtered (I think you will have to log out to see whether your review has been filtered; your review is not filtered from you), you can do something about it. Build some credibility by writing more reviews, get more friends, do not copy-paste any reviews (not even your own!). You may want to read my answer here:
If you are an business-owner and your negative reviews are filtered, thank your stars, but go through each of those. There may be no legitimate reason for them getting filtered, except that the reviewer doesn't have much credibility yet.(see previous para) Unless you are sure that these reviews are from your jealous competitors, take all negative reviews (filtered or unfiltered) as a useful feedback, and try to incorporate that into your operations. However,if it's your 4* and 5* reviews that are filtered, you can't do much, except continuing to deliver good service, so that you get more such reviews!
As a review-reader, always pay attention to filtered reviews, especially for businesses with very few unfiltered reviews. (They will ask you to verify your human identity by filling in a re-CAPTCHA) Read them and use your judgement to see which ones feel right, but take them with a pinch of salt. Sometimes, there is a lot of useful information hidden in the filtered ones.
See question on Quora